Non-poisonous zinc plating baths

ABSTRACT

A non-poisonous zinc plating bath comprising a fundamental plating composition including zinc salt and ammonium salt with or without hydroxy carboxylic acid; a sulfate compound or compounds having the general formula A - B -CH2CH2O2-100SO3M; an alkylnaphthylketon compound or compounds having the general formula   AND A NITROGEN-CONTAINING HETEROCYCLIC RING COMPOUND OR COMPOUNDS SUBSTITUTED WITH CARBOXYL GROUP HAVING THE GENERAL FORMULA   OR MIXTURE THEREOF.

States Hayashida et a1.

aten I 1 [451' Jan. 22, 1974 1 NON-POISONOUS ZINC PLATING BATE-1S 221 Filed: Apr. 7, 1971 211 App1.No.: 132,218

[30] Foreign Application Priority Data Apr. 24, 1970 Japan 45-34717 [52] US. Cl. 2041/55 R, 204/DIG. 2 [51] Int. Cl. C231) 5/12, C231) 5/46.

[58] Field of Search 204/55, DIG. 2

[56] I References Cited UNITED STATES PATENTS 2,813,065 11/1957 Du Rose .1 204/45 3,296,104 1/1967 Eppensteiner 204/55 R 3,296,105 1/1967 Rushmere 3,594,291 7/1971 Todt et a1. 204/55 R 2,389,179 11/1945 Brown 204/49 2,389,181 11/1945 Brown..... 3,655,534 4/1972 Kampe 204/55 Primary ExaminerF. C. Edmundson Attorney, Agent, or Firm-Hammond and Littell [57] ABSTRACT A non-poisonous zinc plating bath comprising a fundamental plating composition including zinc salt and ammonium salt with or without hydroxy carboxylic acid; a sulfate compound or compounds having the general formula A.B -4CH CH Oi- -.mSO M; an alkylnaphthylketon compound or compounds having the general formula R COR1;

and a nitrogen-containing heterocyclic ring compound or compounds substituted with carboxyl group having the general formula or mixture thereof.

5 Claims, No Drawings BACKGROUND OF THE INVENTION This invention relates to novel and improved nonpoisonous zinc plating baths which contain no cyanide therein and which can effectively eliminate occurrence of harmful environmental contamination even when the waste liquids from the baths are discarded out of the system.

One typical prior art zinc plating bath for use in zincplating the surfaces of various base metals has the composition which principally comprises 80 g/l of sodium hydroxide, 35 g/l of Zn and lOO g/l of sodium cyanide and the waste liquid from the plating bath includes cyan-containing substances in a substantially high concentration. Therefore, when the waste liquid, is discarded out of the system into a river or sea, for example, the liquid will seriously contaminate the water in the river or sea.

Of late, low cyan'concentration zinc plating baths containing Zn and sodium cyanide in relatively lower concentrations have been proposed and practically employed. These plating baths generally contain as their principal components 80 g/l of sodium hydroxide, 8 g/l of Zn and 10 g/l of sodium cyanide and in fact, these plating baths, contain sodium cyanide in a lower concentration than the prior art zinc plating baths do.

However, since these recently developed zinc plating baths still contain sodium cyanide therein though the concentration of the cyanide is substantially lower than that of the corresponding component in the comparative prior art zinc plating baths, it is, of course, evident that the low cyan-containing zinc plating baths will still inevitably contaminate the water in a river or sea when the waste liquid from these baths is discarded there. And since the concentration of sodium cyanide in these baths is substantially low, the brilliance-imparting current density range available in these plating baths is limited and current efficiency in these baths is also low to the degree that the productivity of these baths will be substantially reduced. In order to eliminate the difficulties referred to just above, there has been, of quite late, proposed and partically employed a brilliant zinc plating bath which contains no cyanide therein and the brilliant zinc plating bath is disclosed in French Pat. No. 1,524,719. However, it has been found that the brilliant zinc plating bath of the above-mentioned French Patent is still not perfectly satisfactory and especially, is not satisfactory with respect to the brilliance-imparting current density range available.

SUMMARY OF THE INVENTION Another object of the present invention is to provide a non-poisonous zinc plating bath which has a wide range of brilliance-imparting current density available, can be effectively applicable to barrel plating, is excellent in performance and will not cause harmful environmental contamination.

Basically, the non-poisonous zinc plating bath of the present invention generally comprises (a) a conventional zinc plating composition comprising zinc salts and ammonium salt with or without hydroxy carboxylic acid; (b) a sulfate compound or compounds of polyether having the below-mentioned general formula:

wherein A is hydrogen, CI'IHQ" l (n is an integral within the range of 4 20), alkylphenylene, residue after removal of the hydroxy group from molecules of sorbitol or mannitol or residue of alkylpolyamine, B is a bridge selected from the group comprising -COO, -NH, S', O, (OCH CH CH2O)7 and CONH and M is hydrogen, ammonium or a metal selected from the group'comprising, sodium,

potassium, magnesium, calcium and Zinc; (c) an alkylnaphthylketon compound or compounds having the afgcom (2) wherein R is a group selected from the group comprising alkyl, phenyl, methylnaphtyl, styryl, nitrophenylene and aminophenylene and R is a group selected from the group comprising hydrogen, halogen, alkyl, aryl, sulfo, amino, nitro, hydroxyl, carboxy] and formyl; and (d) a nitrogen-containing heterocyclic ring compound or compounds substituted with carboxyl group having the below-mentioned general formula:

ll R \N/ COOH wherein is a heterocyclic ring selected from the group comprising pyridine, quinoline and isoquinoline and R is a group selected from the group comprising hydrogen, halogen, alkyl, alkoxyl, aryl, amino, nitro, hydroxyl, carboxyl, sulfo,- sulfomoyl, substituted sulfomoyl, hydroxycarboxyalkyl, carboxy vinylene, carboxy phenylene, carboxyalkyi, phenylvinylene and mercapto.

EXAMPLES OF THE INVENTION Zinc salts, ammonium salts and hydroxy carboxylic acids to be blended with the other components of the zinc plating baths of the invention may be those usually employed in the conventional zinc plating baths compositions and the zinc salts useful in the baths of the invention include zinc chloride, zinc sulfate and zinc sulfaminate. These zinc salts can be employed in amounts tag in .19, t 2QQs/Llh9a91m msa useful in the baths of the invention include ammonium chloride, ammonium sulfate and ammonium acetate and these ammonium salts can be employed in amounts ranging from 20 to 350 g/l with satisfactory results. The hydroxy carboxylic acids useful in the baths of the invention include maleic acid, citric acid, gluconic acid, tartaic acid and lactic acid and these hydroxy carboxylic acids can be optionally employed in amounts ranging from 1 to 100 g/l.

In preparing the zinc plating baths of the invention, any one of the above-mentioned zinc salts and any one of the above-mentioned ammonium salts are blended together or alternatively, anyone of the abovementioned hydroxy carboxylic acids is further added to the blend. Thereafter, the resulting mixture is added thereto the compounds represented by the general formulas I (2) and (3), respectively, thereto to prepare non-poisonous zinc plating baths. In-order to simplify the disclosure herein and also for better understanding of the invention, the mixture of zinc and ammonium salts with or without hydroxy carboxylic acid will be referred to as the fundamental bath composition hereinafter.

The operative effects of the additive organic compounds represented by the general formulas (l), (2) and (3 respectively when they are added to the fundamental bath composition to prepare a zinc plating bath will be described hereinbelow.

Firstly, the nitrogen-containing heterocyclic ring compound or compounds substituted with carboxyl group having the general formula (3) have buffering action upon the pH of the plating bath thereby to facilitate the pH control of the plating bath.

Secondly, the nitrogen-containing heterocyclic ring carboxylic acid compound or compounds extend the current density range of the plating bath available to impart brilliance to the surfaces of a base metal to be plated and in other words, the compound or compounds serve to reduce the necessary brillianceimparting currency density from a higher value portion thereby to substantially improve the brilliance and covering power of deposit metal on the base metal even when the bath is employed for barrel plating which must be usually carried out at a relatively low current density. An example for manifesting the improvements with respect to both the brilliance and covering power of deposit metal obtainable by the plating bath of the invention with the addition of the nitrogen-containing heterocyclic ring compound or compounds over those obtainable by a control zinc plating bath without the addition of such a compound will be given hereinbelow. When a zinc plating bath comprising 120 g/l of zinc chloride, I80 g/l of ammonium chloride, 20 g/l of sodium gluconate, 0.5 g/l of 2-cthylnaphthylkcton and g/l of CQH11I0+CH2CH2WSO3N3 was employed for zinc-plating a base metal, it was found that the available brilliance-importing current density range stood in a rather high region such as being l620 A/dm and the available range was very limited one as seen from the value shown above.

Such a zinc plating bath cannot be employed forrack and barrel platings which require the brillianceimparting current density range of 0.5- A/dm and 0.1-3 A/dm respectively. However, when the above-mentioned control bath was added thereto 3 g/l of quinaldinic acid which is a typical nitrogencontaining heterocyclic ring compound substituted base metal, it was surprisingly found that the brillianceimparting current density range of the bath containing the carboxylic acid compound was extended to the range of 0.1 2O A/dm and could be suitably employed for plating articles having complicate contours and shapes with satisfactory results. It was also found that the bath with the addition of the carboxylic acid compound could be satisfactorily employed for barrel plating which would generally require a relatively low current density. Now, description will be made of the suitable amount range of a nitrogen-containing heterocyclic ring compound substituted with carboxyl group having such effects to be added to the fundamental bath composition according to the teach of the present invention. When one or more of the above-mentioned heterocyclic ring compound substituted with carboxyl group are employed in amounts less than 0.1 g/l in the bath, the additive acid compound or compounds cannot be expected to exhibit the above-mentioned effects and on the other hand, when the acid compound or compounds are employed in amounts greater than 15 g/l, although the brilliance and covering power of deposit metal obtainable at a relatively low current density will not be adversely affected in any way, there is the tendency to render the deposit metal layer brittle. Thus, the amount range of the additive carboxylic acid compound or compounds is preferably within the range of 0.1 15 g/l. And the pH of the zinc plating bath of the invention is preferably maintained within the range of 3.0 10. The reason is that when the pH is below 3.0, the plated product will be readily subjected to corrosion and the additive carboxylic acid compound or compounds will salt out and on the other hand, when the pH is over 10, the ammonia contained in the bath is allowed to disperse by evaporation. Therefore, the pH of the bath is preferably maintained within the range of 3 10.

The temperature of the plating bath is preferably maintained within the range of l0-50C. Although plating operations employing baths of the nonpoisonous zinc plating compositions of the invention can be performed while the baths being maintained static, better results will be obtained when the baths are air-agitated or by the adoption of the so-called cathode rocker system. Typical nitrogen-containing heterocyclic ring compounds substituted with carboxyl group which are suitably employed in the present invention will be given hereinbelow, and in connection with this it is to be understood that they can be employed either in their simple forms or in the forms of various mixtures thereof.

. 2-methyl-3-carboxy pyridine 5-acetyl-3-carboxy pyridine 2-hydroxy-4-carboxy-6-acetyl pyridine 2-methoxy-3-carboxy pyridine 3-nitro-4-carboxy pyridine 2-amino-4-carboxy pyridine 2-sulfo-4-Carboxy pyridine nicotinic acid 6-chloro-4-carboxy pyridine quinaldinic acid 1 l. 4-carboxy isoquinoline l2. 5-carboxy-8-hydroxy quinoline l3. 7-amino-3-carboxy quinoline l4. 7-acetyl-3-carboxy quinoline l5. 5-mercapto-3-carboxy quinoline Next, description will be made of the operative effects the additive sulfate compounds of polyether having the general formula I) referred to above and to be added to the fundamental bath composition of the baths of the invention. Firstly, the sulfate compound or compounds of polyether accelerate the dissolution of alkylnaphthylketon compound or compounds having the general formula (2) referred to above and to be also added to the fundamental bath composition of the invention into the baths. This action of the sulfate compound or compounds will be further accelerated by selecting ester of sulfuric acid as the end group of the polyether compound or compounds to be employed in the present invention. Furthermore, such an action of the sulfate compound or compounds of polyether serves to increase the salting-out temperature and to facilitate the control of the plating baths.

Secondly, the sulfate compound or compounds of polyether when employed in conjunction with the fundamental bath composition serve to suitably control the electrodeposition of zinc and this is considered due to the fact that a thin colloidal film is probably formed in the cathode because of the structure of +CH CH O #present in the molecules of the polyether compound or compounds.

Thirdly, the sulfate compound or compounds of polyether may serve to control extra-ordinary deposition of zinc at high current densities thereby to prevent the formation of dendrite.

As to the suitable amount of the sulfate compound or compounds of polyether to be added to the baths of the invention, when the sulfate compound or compounds are employed in amounts less than 0.1 g/l, the abovementioned operative effects cannot befully realized and on the other hand, when the amount is over 30 g/l, the obtained deposit metal will be brittleand will be readily subjected to salting-out. Therefore, the sulfate compound or compounds of polyether are preferably employed in amounts from 0.1 to 30 g/l.

Typical sulfate compounds of polyether to be employed in the baths of the present invention will be shown hereinbelow by their chemical formulas. These sulfate compounds may be employed either in their simple forms or in the forms of mixtures thereof with h P ti mpsr t an as. .4

. C H, -COO+CH CH O fi So Na Cg H 7-NH SO Na ,SO Na C H C l-l, O +CH Cl-l O) SO Na CgHn S 50,-;Na

- C gHlEl C 5H1? O- CHQCHIO) SOENR 2-100 Next, description will be made of the operative effects of the alkylnaphthylketon compounds having the general formula (2) referred to above to be added to the zinc plating baths of the present invention. Thealkylnaphthylketon compound or compounds serve to impart brilliance to deposit metal in the plating baths. The addition of the compound or compounds having the general formula (2) is necessary because plating baths comprising the fundamental plating composition having only the sulfate compound or compounds of polyether of the general formula (1) and the nitrogencontaining heterocyclic ring compound or compounds substituted with carboxyl group of the general formula (3) cannot produce bright deposit metal and will produce semi-bright deposit metal at the best.

However, it has been found that when an alkylnaphthylketon compound or compounds are added to the zinc plating bath comprising the above-mentioned fundamental composition having the compound or compounds of the general formula (1) and the compound or compounds of the general formula (3) added thereto, a surprising bright plating can be attained. For example, when a zinc plating bath was prepared by adding 0.8 g/l of an alkylnaphthylketon compound to the fundamental composition having a sulfate compound of polyester in an amount in the range referred to above and a heterocyclic ring compound substituted with carboxyl group in an amount inthe ranged referred to above added thereto it was possible to produce a bright plated product whose brilliance substantially corresponded to that of a bright nickel-plated product. The brilliance obtained by the use of the alkylnaphthylketon compound as one of the three types of additives was confirmed as to be due to the presence of the naphthalene in the additive compound through the below-given comparison experiments. In some of the experiments, zinc plating baths were prepared by adding acetophenone in the amount range from 0.1 to 20 g/l to the fundamental composition including a sulfate compound or compounds of polyether in the abovementioned amount range and a heterocyclic ring compound or compounds substituted with carboxyl group in the above-mentioned amount range as taught in the above-mentioned French patent, but the brillianceimparting current density range of these baths was narrow such as being 0.5 2 A/dm whereas in the others of the experiments, plating baths were prepared by adding an alkylnaphthylketon compound or compounds in the above-mentioned amount range to the fundamental composition including the compound or compounds of the general formula (1) and the compound or compounds of the general formula (3) without the use of acetophenone and the brilliancem122tt ns q nt sistt it nsz .that? bathin 9 wider such as being 0.1 to 20 A/dm and bright plated products were obtained over. As to the suitable amount of an alkylnaphthylketon compound or compounds to be added to the plating baths of the invention, when the 8 l2. 1-hydroxy-4-chl6r6 2 a etonaphihbri 13. 1-carboxy-5-acetonaphthone 14. -formyl-l-acetonaphthone 15. 3-amino-2-methoxy-l-acetonaphthone compound or compounds having the general formula 5 1n the foregoing, description has been made of the (2) are employed in amounts less than 0.1 g/1,the briloperative effects of the various additives to be emliance-imparting effects of the baths are insufficient ployed in conjunction with the fundamental plating and on the other hand, when the compound or combath composition according to the present invention, pounds of the general formula (2) are employed in but in order to manifest the excellent operative effects amounts in excess of 5.0 g/l, the obtained deposit metal of the baths having the above-mentioned composition will be brittle, Th r f e, the co ound or compounds of the invention over those obtainable by the prior art of the general formula 2) should be employed in the zinc plating baths, the following comparison experirange from 0.01 to 5.0 g/l. Typical examples of alkylments were conducted using the baths of the invention naphthylketon compounds usefull l d i th and those of the prior art. In any of these experiments, zinc plating baths of the invention are given hereinbethe fundamental ComPOsltion Comprised 90 8 of Zinc low and they can be employed either in their simple Chloride, 130 g/ of ammonium o de and g/ of forms or in the forms of mixtures thereof without detartaric acid and the undam ntal composition was parting from the spirit and scope of the invention with added thereto the additives as shown in the belowthe same results. given Table 1 (prior art) and Table 2 (the invention), 1. 5-chl0ro-2-acet0naphth0ne 20 respectively to prepare the zinc plating baths. Test 2. 5-methyl-2-acet0naphth ne pieces were zinc-plated using the thus prepared baths 3. S-amino-l-acetonaphthone under the conditions such as the temperature of 30C, 4. 1,5-diacetonaphthone the current density of 3 A/dm and pH of 5.0 while agi- 5. acetonaphthone (l or 2) tating the baths with air. The zinc-plated test pieces 6. 2-phenylnaphthylketone were measured or observed in accordance with the 7. 1,1 '-dinaphthy1ketone Hull cell test procedure for the leveling and appearance 8. l-stylylnaphthylketone of the plated products and the brilliance-imparting cur- 9. p-aminophenyl-l-naphthylketone rent density and of the baths. The results of these mea- 10. p-nitrophenyl-2-naphthone surement or observation are given in the Table 1 and 1 1. 1-hydroxy-4-acetonaphthone Table 2, respectively.

Table 1 Results obtained by the use of prior art zinc plating baths Brillibrillithrowing ance inanceadditive power dex of impart- No. or reflccing cuadditives (71.) tion rrent appearance density range W (g/ (A/dm) 1 None 0 43 3 lustreless rough dendrite 2 C.,H4(OH )COONa 2 5 A lustreless T CHJCOONu') 2 4 I I Lustreless 3 4| 7 no rough dend- N I7 fi -ltrite in high 10 current density 2 z )?m' H region -C,,1 I (5CZT1I CH 'O I '2 Tight amber 4 44 15 0.5 2.0 color brilliance SI IB B J Y in low current 10 density portion (CH CH,O) H

C H QCGOF 2 5 45 17 0.5 2.0

c..H,.. C..H,-0

[Trans. Am. Electrochem. SOC. 313 (1923)].

The values of the throwing power given in the 5150; Table were obtained in accordance with the HarirEHIEQand Blum W Procedure 2 The values of the brilliance (index of reflection) given in the above Table were obtained by the use of a precision photoelectric color meter produced by the Tokyo Shibaura Electric K.K. of Tokyo.

amperes.

1 The appearance of the processed test pieces were observed after 5 minutes of immersion thereof in the baths with the total current of 3 TABLE 2 (Results obtained by the use of the zinc plating baths of the invontion) Brilliance (imparting Throw- Brilliance current ing (index of density v power, reflection) mngei Number AddltlVeS (g.l1.) percent percent A.,d1n.= Appearance 1 CQH Q -O-(CHZCH2O)|5 03N8 v i? 0 OH 2 I CH3 Brilliance on 44 88 0. 2-20 the whole N surface.

1 C 0 CH3 2 CsHi7COO-(CHzCH20)anS OaNa 10 C O OH 2 1 C 0 CH3 3 Sulfuric acid ester of polyoxyethylene sorbitoi steiuate 10 O CH:

0 CHa- N 4a 84 0. 1-20 Do.

4 CqHm-CuHr-S(GH2CH20)ao-S OaNa 10 (ii 0 OH 3 -O CHa N 43 85 0. 1-20 Do.

2 C O 0 Ha 5 C8H17-CONH(CH2GH20)15S OaNa 10 (I O OH 2 C 0 0 Ha From the data given in the above Tables 1 and 2 it will be understood that the zinc plating baths employing the three types of additives of the invention can produce substantially higher brilliance and surprisingly wider brilliance-imparting current density ranges as compared with those obtainable by the control prior art zinc plating baths.

For further better understanding of the present invention, serveral examples in which the present invention was successfully carried out will be given hereinbelow. However, it should be understood that the invention is not limited to the precise examples in any way.

EXAMPLE 1 A zinc plating bath was prepared by adding 10 g/l of cgH g SO Na as a Sulfate Compound of polyether, 3 g/l of 2-methyl-3- carboxyquinoline as a carboxylic acid compound of nitrogen-containing heterocyclic ring and 0.6 g/l of 5-chloro-2-acetylnaphton as an alkylnaphthylketon compound to the fundamental composition comprising 120 g/l of zinc chloride and 190 g/l of ammonium chloride and a zinc plating operation was conducted using the bath under the conditions such as the temperature of 25C, the pH of 5.0 and the current density of 5 A/dm" for minutes. The obtained deposit metal had a high brilliance.

EXAMPLE 2 A zinc plating bath was prepared by adding 8 g/l of EXAMPLE 3 A zinc plating bath was prepared by adding 10 g/l of C,,H, +OCH CH CH O)-(CH CH OM SO Na, 3 g/l of nicotinic acid and 2.5 g/l of B-sulfo-Z-acetylnaphton to the fundamental plating composition comprising 200 g/l of zinc sulfate, 30 g/l of ammonium chloride and g/l of sodium salt of maleic acid and a zinc plating operation was conducted using the bath under the conditions such as the temperature of C, the pH of 1,5

and the current density of 3 A/dm" for 10 minutes. The obtained deposit metal had a high brilliance.

EXAMPLE 4 A zinc plating bath was prepared by adding l5 g/l of carboxyisoquinoline and 0.9 g/l of S-methyl-B- acetonaphton to the fundamental composition comprising 40 g/l of zinc chloride and 230 g/l of ammonium chloride and a zinc plating operation was conducted using the bath under the conditions such as the temperature of 35C. the pH adjusted to 7.2 with ammonia water and the current density of 2 A/dm for l0 minutes. The obtained deposit metal had high brilliance and softness.

1 2. EXAMPLE 5 A zinc plating bath was'prepared by adding 5 g/l of H O (CH CH SO Na, 3 g/l of HS (CH- C- H 055 SO Na and 3 g/l of 4-carboxyisoquinone and 0.3 g/l of 5chloro-B-acethyl-naphton to the fundamental composition comprising 30 g/l of zinc chloride and 250 g/l of ammonium chloride and a zinc plating operation was conducted using the bath under the conditions such as the temperature of about 30C, the pH of about 5.0 and the current density of 2 A/dm for 10 minutes. The obtained deposit metal had a high brilliance.

While there have been described what are at present considered to be the preferred examples of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is aimed, therefore, in the appended appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. An aqueous cyanide-free zinc electroplating bath containing (a) a water soluble zinc salt selected from the group consisting of zinc chloride, zinc sulfate and zinc sulfaminate, a water-soluble ammonium salt selected from the group consisting of ammonium chloride, ammonium sulfate and ammonium acetate, and optionally a hydroxycarboxylic acid; (b) from 0.1 to 30 gm/liter of at least one sulfate compound of the formula wherein A is selected from the group consisting of hydrogen, C,,H with n being an integer of 4 to 20, al-

kylphenyl, sorbitol, mannitol, alkylpolyamine, B is a member selected from the group consisting of H O) and CONH, and M is selected from the group consisting of hydrogen, ammonium, alkali metal, alkaline earth metal and zinc; (c) from 0.01 to 5.0 gm/liter of at least one alkylnaphthylketone of the formula ll R \N/ COOH wherein is a heterocyclic ring selected from the group consisting 14- of 5151151; in which s aid bath confiifi sTro mfi3 to 2f5 gm/liter of said alkylnaphthylketone.

4. The aqueous cyanide-free zinc electroplating bath of claim 1, in which said bath contains from 3 to 5 gm/liter of said heterocyclic ring compound.

5. The aqueous cyanide-free zinc electroplating bath of claim 1, in which the pH of said bath ranges from 3 to 10. 

2. The aqueous cyanide-free zinc electroplating bath of claim 1, in which said bath contains from 8 to 15 gm/liter of said sulfate compound.
 3. The aqueous cyanide-free zinc electroplating bath of claim 1, in which said bath contains from 0.3 to 2.5 gm/liter of said alkylnaphthylketone.
 4. The aqueous cyanide-free zinc electroplating bath of claim 1, in which said bath contains from 3 to 5 gm/liter of said heterocyclic ring compound.
 5. The aqueous cyanide-free zinc electroplating bath of claim 1, in which the pH of said bath ranges from 3 to
 10. 